1. Field of the Invention
The present invention relates to a transmitting circuit apparatus comprising a quadrature modulator used in wireless communications and the like.
2. Related Art of the Invention
In a transmitting circuit apparatus used in digital wireless communications by a modulation scheme such as QPSK, a quadrature modulator i s generally used as a modulator. FIG. 15 shows the basic configuration of a prior art transmitting circuit apparatus. In FIG. 15, numeral 403 indicates a quadrature modulator. Numeral 404 indicates a band-pass filter. Numeral 405 indicates an I-Q signal generator. Numeral 406 indicates a local oscillator. Numeral 407 indicates a phase shifter. Numerals 408 and 409 indicate mixers. Numeral 410 indicates a synthesizer. Numeral 411 indicates a power amplifier. The quadrature modulator 403 is composed of the phase shifter 407, the mixer 408, the mixer 409, and the synthesizer 410. The I-Q signal generator 405 outputs a baseband I signal and a baseband Q signal which are analogue signals, and then inputs them to the quadrature modulator 403. The local oscillator 406 outputs a sine wave signal at a carrier frequency. The sine wave signal is distributed into two signals each having a phase different from each other by 90 degrees by the phase shifter 407, and then the two signals are input to the mixer 408 and the mixer 409, respectively. The mixer 408 and the mixer 409 perform amplitude modulation on the respective signals each being at the carrier frequency and having a phase different from each other by 90 degrees by using the baseband I and Q signals. The modulated signals are synthesized by the synthesizer 410 into an output signal of the quadrature modulator 403. The output signal of the quadrature modulator 403 is amplified by the power amplifier 411, and then undergoes reduction of unnecessary frequency components by the band-pass filter 404, thereby being output.
FIG. 16 shows another example of a prior art transmitting circuit apparatus used in an optical base station for mobile communications and the like. In this configuration of the optical base station, in order to permit the use of wireless terminals even in underground malls and the like where the radio waves from a master station is not reachable, a master station having all the control functions of a base station is connected via an optical fiber to a slave station serving as a front end for wireless signals.
The configuration of FIG. 16 is basically similar to that of FIG. 15 except that the quadrature modulator 403 and the power amplifier 411 are interconnected via an optical fiber 425. Thus, like numerals are assigned to the like parts, and the detailed description is omitted. In FIG. 16, numeral 421 indicates a master station. Numeral 422 indicates a slave station. Numeral 423 indicates an E/O converter. Numeral 424 indicates an O/E converter. Numeral 420 indicates an antenna.
In the master station 421, the output signal of the quadrature modulator 403 is converted from an electric signal to an optical signal by the E/O converter 423 composed of a laser diode, and then transferred through the optical fiber 425 to the slave station 422. In the slave station 422, the received optical signal is converted to an electric signal by the O/E converter 424 composed of a photodiode, and then amplified by the power amplifier 411. After that, the signal undergoes reduction of unnecessary frequency components by the band-pass filter 404, and then is transmitted from the antenna 420.
In such prior art transmitting circuit apparatuses described above, the input signal to the quadrature modulator 403 is analogue, and hence the mixers 408, 409 need to be free from distortion. Accordingly, it is difficult to sufficiently increase the output level of the quadrature modulator 403. Thus, the power amplifier 411 is used for amplification, however, the power amplifier 411 also needs to be operated in the linear range causing only smaller distortion. This requires the operation at levels sufficiently lower than the saturation level. As a result, the power consumption of the power amplifier 411 has been rather large, and hence has prevented the reduction of overall power consumption of such a transmitting circuit apparatus.
Further, in the configuration of FIG. 16 which is another example of a prior art transmitting circuit apparatus used in an optical base station, in addition to the large power consumption of the power amplifier 411, linearity is required also for the E/O converter 423, the optical fiber 425, and the O/E converter 424. Accordingly, in spite of the simple configuration of the slave station, the linearity is difficult to ensure, and there is a problem of large power consumption.